plastic pants 

Plastic pants (also known variously as "baby pants", "vinyl pants", "plastic panties", "waterproof pants/panties", "incontinent pants/panties" and, less commonly, as "rubber pants" or "diaper pants") are closed-crotch panties, resembling ladies/girls panties, that are designed to be worn over cloth or disposable diapers to help prevent leaking when diapers are wet or soiled. Plastic pants are generally made of PVC (vinyl), but the name is sometimes also applied to like garments made of rubberized nylon or polyester.
Plastic pants may be worn over diapers, training pants, or regular underwear by babies, toddlers, children and adults of all ages who are incontinent or enuretic (bedwetter).

Carol bought plastic pants to use over the baby's diapers.
Louise's husband is incontinent and wears plastic pants over diapers at night to keep the bed dry.
Jimmy's mother is potty training him using training pants, but she wisely puts plastic pants over them in case of "accidents".

plastic pants by Ken M. H. October 9, 2007